The invention relates to injection molding and more particularly to an improved method of manufacturing a metallurgically monolithic integral elongated heated probe.
Heated probes which extend into a melt passage to heat the melt flowing around them are well known in the art. It is also known to make these probes by integrally casting a heating element into a steel body, but all of the previous methods have one or more disadvantages. For instance, the applicant's U.S. Pat. No. 4,376,244 which issued Mar. 8, 1983 discloses a method of casting a copper slug around a cartridge heater from the rear, but it has the disadvantage that two separate heating steps in the vacuum furnace are required rather than only one. The applicant's U.S. Pat. No. 4,777,348 which issued Oct. 11, 1988 similarly relates to copper and requires two separate heating steps and also requires drilling a diagonal filler duct from the filler tube. While it shows the body of the probe being machined with a separate nose portion, they are both made of H13 tool steel which is not sufficiently abrasion resistant in the tip area for some applications. In order to cast the heating element in copper, the probe must be heated above the melting temperature of copper at 1981.degree. F. This has the disadvantage that it is above the austenitizing range of the steel body and results in it having lower yield strength. Also, the filler tubes were brazed or welded in place and had to be machined off and so were not reusable.
More recently, the applicant's Canadian patent application serial number 2,032,728-6 filed Dec. 19, 1990 hentitled "Injection Molding Probe with Varying Heat Profile" discloses a method of integrally brazing the heating element in a nickel alloy. In addition to being filled from the forward end, this method has the disadvantage that the alloy is not sufficiently conductive to provide the very rapid cycle times required to be competitive. The applicant's Canadian patent application serial number 2,037,186-2 filed Feb. 28, 1991 entitled "Injection Molding Probe with a Longitudinal Thermocouple Bore and Off Center Heating Element" similarly has the disadvantage that the heating element is cast in an alloy which is not sufficiently conductive for some applications.